1. Field of the Invention
The present invention relates to paper-making machines, and, more particularly, to an ultrasonic deflocculating device for use in paper-making machines.
2. Description of the Related Art
A paper-making machine is used for making a fiber web, such as a paper web, from a fiber suspension. The fiber suspension is typically in the form of fibers, such as wood fibers, which are suspended in water. The fiber suspension is introduced into a headbox, such as a valley (TM) or a hydraulic headbox, at the wet end of the machine. The headbox discharges the fiber suspension from an outlet of a discharge nozzle which causes the exiting fiber suspension to have a particular cross-sectional profile. The fiber suspension is transferred onto a forming fabric on the wet end of the machine as it leaves the headbox.
It is necessary to treat the fiber suspension which is introduced into the headbox such that the fiber web produced as the end product is of a particular quality with minimal defects. Fox example, the fiber suspension may be cleaned and bleached prior to introduction into the headbox. Another factor associated with the fiber suspension which tends to affect the quality of the fiber web is the tendency of the fibers within the fiber suspension to clump together, which may also ultimately result in clumps of fibers within the produced fiber web. Such clumping is believed to be a result of hydro entanglement of the fibers during which the fibers spin together and become tangled. The clumped fibers are not solid particles, but rather are tangled fibers. The tendency of the fibers within the fiber suspension to clump together is known as "flocculation" of the fiber suspension.
It is known to place a mechanical device within the headbox for the purpose of agitating the fiber suspension and thereby deflocculating the fiber suspension. For example, one or more rotatable rolls may be placed within the headbox. Such rolls, known as distributor rolls, typically are in the form of hollow rolls with a plurality of perforations at the periphery thereof. Rotation of the distributor rolls agitates the fiber suspension and deflocculates the fiber suspension within the headbox.
Further, it is known to place a vertical partition within a headbox for the purpose of deflocculating the fiber suspension. For example, U.S. Pat. No. 5,277,765 (Graf), assigned to the assignee of the present invention, discloses a vertical partition with a cross-sectional profile which provides effective deflocculation of the fiber suspension within the headbox within a range of flow rates of the fiber suspension.
With the use of the above-mentioned mechanical deflocculation devices, flow disturbances such as so-called "eddy currents", or "eddies", or microcurrents, occur within the fiber suspension at the walls of the headbox. Eddies are caused by adhesion between the walls and the fiber suspension, and may be detrimental to paper quality.
Another problem with the above-mentioned deflocculation devices is that reflocculation occurs in the fiber suspension before it exits the discharge nozzle of the headbox. Known deflocculation devices as described above are placed either within the inlet portion or the main chamber of the headbox. However, reflocculation can occur after deflocculation within a fraction of a second after the fiber suspension flows out of the effective range of the deflocculation device. The fiber suspension flows through the headbox at a rate which is slow enough to allow the fiber suspension to reflocculate before leaving the headbox. Known deflocculation devices may therefore not totally prevent the formation of "flocs" in the paper web.
It is also known to place an ultrasonic device within an explosion block associated with an inlet of a headbox. The ultrasonic device, in conjunction with the explosion block, functions to suspend solid particles such as clays, fillers, fines, colors, pigments, and/or mercury within a fiber suspension in a paper-making machine. The ultrasonic device and explosion block conjunctively break up the solid matter into very fine particles which may be then adequately suspended within the fiber suspension. Such an ultrasonic device is not used for deflocculation of the fiber suspension, and in fact would not even inherently work for effective deflocculation of the fiber suspension. That is, reflocculation of the fiber suspension would quickly occur within the headbox since such an ultrasonic device is placed at the inlet to the headbox, as indicated above with respect to mechanical deflocculation devices.
What is needed in the art is a device which deflocculates the fiber suspension immediately prior to discharge from the headbox such that reflocculation does not occur, and which does not obstruct the flow of the fiber suspension through the discharge nozzle or create eddies.